1. Field
The present invention relates to the field of shaft angle encoders. More specifically, the present invention relates to a high resolution rotational shaft angle encoder particularly well adapted to detect the rotation of the inner gimbal of gyroscope.
2. Art Background
Gyroscopes:
Attitude indicators in aircraft, known as artificial horizons, use two-degree-of-freedom gyroscopes for inertia space reference and the measurement of pitch and roll relative to the gravitational vector. The gravity vector is approximated by a pendulous device (suspended weight) which indicates the apparent vertical, that is, the combined effect of gravity and acceleration. Such a device, as described in Gyroscopic Theory, Design, and Instrumentation, 1980, Wrigley, Hollister and Denhard, The M.I.T. Press, Cambridge, Mass., uses the average direction of the apparent vertical over a period of time to approximate the direction of gravity. The inner element, or gimbal, rotates on two axes with respect to the frame. Since the only contacts between the housing and the outer gimbal and the outer gimbal and inner gimbal are through the bearings, it is difficult to provide both power to the inner gimbal for the motor and to extract angle information from shaft angle encoders without limiting the range of motion of the gyroscope. Accordingly it is desirable to provide an accurate and inexpensive shaft angle encoder for measuring the angle of rotation of the inner gimbal about its axis of rotation without interfering with the operation of the gyroscope.
Shaft Angle Encoders:
High resolution shaft angle encoders as used in theodolites typically use electro-optical phase quadrature detectors such as described in U.S. Pat. No. 4,346,989, titled Surveying Instrument, issued to Alfred F. Gort and Charles E. Moore Aug. 31, 1982 and assigned to the Hewlett-Packard Company. While these systems have quite high resolution, they are not adapted for use in measuring the angle of rotation of the inner gimbal about its axis of rotation without interfering with the operation of the gyroscope. Further, conventional phase quadrature sensing systems, referred to herein as dual-path systems, employ two separate optical paths which are spaced apart and interact with different areas of an optical pattern. As a result, small variations in the pattern from one area to another, such as line width variations resulting from uneven thermal expansion, introduce undesirable errors. Further still, dual-path system employ two separate detectors, each of which has an associated phase plate. Any misalignment of these two phase plates relative to each other introduces an error into the shaft angle encoder and further reduces its resolution. Accordingly, it is desirable to provide an electro-optical phase quadrature detector adapted for use in measuring the angle of rotation of the inner gimbal of a gyroscope. Further, it is desirable to provide a shaft angle encoder that is not subject to the errors introduced by the use of dual optical paths.